A new ink jet printing technology has been described in WO-A-9311866 in the name of Research Laboratories of Australia Pty Limited which was published on the 24th Jun. 1993. This process provides a means of producing variable sized droplets that contain a high concentration of particulate material. Specific advantages conveyed by this process include the ability to form droplets as small as a few micrometres while still using pigments as the colorant material. This is because the size of the droplets are controlled primarily by the voltage on an injection point plus the ability of the particles to be charged and so are not limited by the size of an ink jet nozzle. Also the colorant material is significantly concentrated in the ejected droplets. Therefore, high resolution in high density images based on light and water resistant particles can be produced.
It is known that for good performance ink used in the abovementioned ink jet devices should have a volume resistivity in excess of 10.sup.9 ohm.cm to allow particle ejection driven droplet formation. In addition, the conductivity of the ejecting particle must be sufficiently low so as to maintain high ink resistivity. Inks which contain a high level of conductive particles tend to be difficult to print with and show poor print stability. It is thought that this is largely because high local concentrations of these conductive particles are able to form spatially extended conductors within the system and whenever these form in the wrong places they can impair the system.
There are several commercially important marking particles that exhibit high electrical conductivity and are therefor not well suited for optimum performance in the ink jet process described in patent publication WO-AWO 9311866. The most important of these include carbon black, magnetic iron oxide and metallic powders but the invention is not restricted to these particles and their use as pigments in inks.
Carbon black (Cl Pigment Black 7) is the most widely used black pigment in conventional printing technologies. Carbon black pigments are prepared by the incomplete combustion of organic (carbon containing) fuels. The pigment usually consists of elemental carbon in combination with residual volatile material of up to 20%, the exact composition being dependent on the fuel stock used and the method and conditions of manufacture. The surface characteristics of carbon black including the conductivity or resistivity of the particles appear to be largely dependent on the amount and type of volatile material present.
Some of the desirable properties of carbon black are that it has excellent opacity, It has a neutral black colour, it has excellent resistance to acid alkali soap and solvent, it is extremely light fast and it is relatively inexpensive.
These features make carbon black very desirable as a marking particle for many printing technologies, however, carbon black has an inherent high conductivity and the performance of carbon black inks in the abovementioned printing technology is therefore less than optimum.
Magnetic iron oxide occurs naturally as mineral magnetite. Alternatively it can be synthesised by a variety of processes such as the precipitation of hydrated ferric oxide from a solution of iron salts followed by dehydration and then reduction with hydrogen. This black pigment material is characterised by a strong permanent magnetism. Commercial uses of magnetic iron oxide include the manufacture of magnetic inks for the printing of MICR information (magnetic ink character recognition).
These magnetic iron oxide particles have a high conductivity and again while they are useful as marking particles their performance in the abovementioned ink jet printing technology is not optimum.
Metallic powders consist of metals or alloys of metals. Examples are Cl Pigment Metal 2 which is an alloy of copper and zinc and Cl Pigment Metal 1 which is a powdered aluminium. Applications for the printing of metallic powders include decorative marking and the printing of electrically conductive circuits. It will be realised of course that metallic powders are inherently conductive and as such do not provide optimum printing using the abovementioned ink jet printing technology.
As mentioned above the present invention is not limited to these particular particles.
It is an object of this invention to provide a method of reducing the conductivity of such pigment particles and to making inks using such reduced conductivity or increased resistivity particles.